Sediment-induced buoyancy destruction and drag reduction in estuaries

This paper presents an analysis of drag reduction by buoyancy destruction in sediment-laden open channel flow. We start from the log-linear profile proposed by Barenblatt (Prikladnaja Matematika i Mekhanika, 17:261274, 1953), extended with a second length scale to account for free surface effects. Upon analytical integration over the water depth, an expression for sediment-induced drag reduction is found in terms of an effective Chezy number, water depth, bulk Richardson number, and Rouse number. This relation contains one empirical/experimental coefficient, which was obtained from a large series of numerical experiments with a 1DV POINT MODEL. Upon calibration of this model against field and laboratory observations, we tuned the turbulent Prandtl-Schmidt number and found an optimal value of sigma(T)=2, consistent to observations by Cellino and Graf (ASCE, J Hydraulic Engineering, 125:456-462, 1999). All numerical results could be correlated with the simple relation C-eff - C-0 + 4 root ghRi(*)beta, which is valid for fine sediment suspensions under conditions typical in open channel flow.